1) Field of the Invention
The present invention relates to compensating for a color shift that occurs at a time of image transfer, in an image forming apparatus, such as a plain paper reproducing machine, a plain paper facsimile machine, a page printer, and the like, that produces multi-color images.
2) Description of the Related Art
In an image forming apparatus of a so-called tandem type, a multi-color image is produced by transferring images formed on a plurality of photosensitive drums in a superimposing manner. In such image forming apparatus, dynamic transfer color shift is caused due to factors such as angular speed fluctuation of a photosensitive drum occurring at a gear or a joint, speed fluctuation of a transfer belt, eccentricity of the photosensitive drum, and speed unevenness between belt positions occurring due to stretching of the transfer belt, which cause positional deviations among transfer patterns corresponding to the respective photosensitive drums, and these positional deviations finally appear as a color shift.
Of the above causes, the eccentricity of the photosensitive drum generates a positional deviation corresponding to one rotation of the drum. However, Japanese Patent Application Laid-Open No. H9-146329 discloses a countermeasure where phases of eccentricities of a plurality of photosensitive drums are properly adjusted so that, even if positional deviation occurs in respective photosensitive drums, color shift among the photosensitive drums is prevented from occurring.
As a method for detecting eccentricity of a photosensitive drum, Japanese Patent Application Laid-Open No. 2001-339972 discloses a technology that uses a displacement gauge of a contact type and one of a non-contact type. However, the gauge of the contact type is not reliable, and the gauge of the non-contact type (an optical system) has a complicated configuration.
Japanese Patent Application Laid-Open No. H9-146329 discloses that when eccentricities of respective photosensitive drums are adjusted, a method of detecting the eccentricities is important. Patterns for detecting positional deviation, each being constituted of a toner image, that are formed on a transfer belt (an endless belt), are sequentially sampled by optical sensors, to detect positional deviation data in a sub-scanning direction, and information about the eccentricities.
However, since the positional deviation data in the sub-scanning direction obtained by sampling basically includes a plurality of positional deviations caused by the transfer color shifts, it contains rather complicated waveforms. Therefore, it is very difficult to separate or extract information about a cyclic positional deviation corresponding to one rotation of the photosensitive drum from the positional deviation information, and the separated information is not very accurate.
The positional deviation corresponding to a cycle of one rotation of the photosensitive drum appears as synthesis of a positional deviation due to eccentricity of the photosensitive drum itself, and a positional deviation due to an angular speed fluctuation of a shaft of the photosensitive drum. Moreover, a phase thereof is different from a phase due to the eccentricity of the photosensitive drum itself. Therefore, even if the phase of the cyclic positional deviation corresponding to one rotation of the drum is detected, the phase of the drum eccentricity cannot be obtained.
The eccentricity of the photosensitive drum causes a positional deviation in a sub-scanning direction and a positional deviation in a main scanning direction, and it is preferable to reduce both the deviations. The details of a generating mechanism of the deviations is explained later, but the deviations occur due to fluctuation of the writing position due to the eccentricity of the photosensitive drum, because a writing beam is obliquely incident on a surface of the photoconductor.
Even if the phase of the positional deviation in the sub-scanning direction is not grasped, it is possible to correct the positional deviation in the sub-scanning direction by synthesizing the deviation with the positional deviation due to the angular speed fluctuation in the same cycle. However, the positional deviation in the main scanning direction cannot be corrected unless a phase thereof is grasped.
The main scanning direction herein is the same direction as image writing direction of writing an image to the photosensitive drum, and is parallel to a direction in which a rotational shaft of the photosensitive drum extends. The sub-scanning direction is perpendicular to the main scanning direction on an image plane, and corresponds to a conveying direction of a transfer belt. These definitions are applied in the following explanation.